图形验证码

 public class SecurityCode
    {
        /// <summary>
        /// 生成随机的字符串
        /// </summary>
        /// <param name="codeCount">验证码长度</param>
        /// <returns></returns>
        public string CreateRandomCode(int codeCount)
        {
            string allChar = "2,3,4,5,6,7,8,9,A,B,C,D,E,a,b,c,d,e,f,g,h,i,g,k,m,n,p,q,r,F,G,H,I,G,K,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,s,t,u,v,w,x,y,z";
            string[] allCharArray = allChar.Split(',');
            string randomCode = "";
            int temp = -1;
            Random rand = new Random();
            for (int i = 0; i < codeCount; i++)
            {
                if (temp != -1)
                {
                    rand = new Random(i * temp * ((int)DateTime.Now.Ticks));
                }
                int t = rand.Next(35);
                if (temp == t)
                {
                    return CreateRandomCode(codeCount);
                }
                temp = t;
                randomCode += allCharArray[t];
            }
            return randomCode;
        }

        /// <summary>
        /// 创建验证码图片
        /// </summary>
        /// <param name="validateCode">验证码无干扰字符串</param>
        /// <returns></returns>
        public byte[] CreateValidateGraphic(string validateCode)
        {
            Bitmap image = new Bitmap((int)Math.Ceiling(validateCode.Length * 22.0), 40);
            Graphics g = Graphics.FromImage(image);
            try
            {
                //生成随机生成器
                Random random = new Random();
                //清空图片背景色
                g.Clear(Color.White);
                //画图片的干扰线
                for (int i = 0; i < 25; i++)
                {
                    int x1 = random.Next(image.Width);
                    int x2 = random.Next(image.Width);
                    int y1 = random.Next(image.Height);
                    int y2 = random.Next(image.Height);
                    g.DrawLine(new Pen(Color.Silver), x1, x2, y1, y2);
                }
                Font font = new Font("Arial", 20, (FontStyle.Bold | FontStyle.Italic));
                LinearGradientBrush brush = new LinearGradientBrush(new Rectangle(0, 0, image.Width, image.Height), Color.Blue, Color.DarkRed, 1.2f, true);
                g.DrawString(validateCode, font, brush, 3, 2);


                //画图片的前景干扰线
                for (int i = 0; i < 100; i++)
                {
                    int x = random.Next(image.Width);
                    int y = random.Next(image.Height);
                    image.SetPixel(x, y, Color.FromArgb(random.Next()));
                }
                //画图片的边框线
                g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);

                //保存图片数据
                MemoryStream stream = new MemoryStream();
                image.Save(stream, ImageFormat.Jpeg);

                //输出图片流
                return stream.ToArray();
            }
            finally
            {
                g.Dispose();
                image.Dispose();
            }
        }
    }

/// <summary>
        /// 图形验证码
        /// </summary>
        /// <returns></returns>
        public IActionResult GetActionSecurityCode()
        {
            var code = new SecurityCode();
            var securityCodeInput = code.CreateRandomCode(5);
            HttpContext.Session.SetString("image", securityCodeInput.ToLower());
            HttpContext.Session.CommitAsync();
            byte[] buf = code.CreateValidateGraphic(securityCodeInput);
            return File(buf, "image/Jpeg");

        }

<img id="validCode" alt="" src="/Register/GetActionSecurityCode" title="换一张" onclick="RefValid()" style="width:130px;height:39px;cursor:pointer" />

 function RefValid() {
        $("#validCode").attr("src", "/Register/GetActionSecurityCode?id=" + new Date());
    }